Effect of Molybdenum on Microstructure and Properties of Plasma Cladded Cobalt-Based Alloys - Literature Study Note

Literature Overview

This research by Hou Qingyu and Huang Zhenyi from the Key Laboratory of Metal Materials and Processing, School of Materials Science and Engineering, Anhui University of Technology, published in Rare Metals in 2006 under funding from the Anhui Provincial University Young Teacher Research Fund (Project 2006jql082), investigates the systematic effects of molybdenum addition on the microstructure and mechanical properties of plasma transferred arc (PTA) cladded cobalt-based alloys. Cobalt-based alloys, particularly the Stellite family, are among the most widely used materials for severe wear and corrosion applications, and their performance is critically dependent on alloy composition and microstructural control. The study provides valuable insights into the role of molybdenum as an alloying element in optimizing the performance of PTA-cladded cobalt-based overlays.

Core Technical Content and Interpretation

Cobalt-Based Alloy System Fundamentals

Cobalt-based alloys (Stellite-type) exhibit exceptional wear resistance, corrosion resistance, and high-temperature strength due to their unique combination of FCC matrix and hard carbide phases. The base composition typically includes:

• Cobalt (Co): 55–65 wt% — provides FCC matrix, high-temperature strength, and corrosion resistance
• Chromium (Cr): 25–30 wt% — solid solution strengthening, oxidation resistance, and carbide formation
• Tungsten (W): 10–15 wt% — solid solution strengthening and carbide formation (WC, W₇C₃)
• Carbon (C): 1.0–1.5 wt% — carbide former for hardness and wear resistance
• Iron (Fe): Balance — diluent, cost reduction

Molybdenum is added as a secondary alloying element to further enhance specific properties. The study systematically investigates Mo additions in the range of 0–8 wt% to establish composition-property relationships.

Molybdenum's Metallurgical Functions

Molybdenum exerts multiple effects on the microstructure and properties of cobalt-based alloy overlay deposits:

1. Solid solution strengthening: Mo atoms in the FCC Co matrix provide significant lattice strain hardening due to the size mismatch (Mo atomic radius 139 pm vs. Co 125 pm). Each wt% Mo contributes approximately 15–20 HV to the matrix hardness.
2. Carbide modification: Mo participates in carbide formation, modifying the type, size, and distribution of carbides:

• At low Mo (< 2 wt%): Mo substitutes for W in WC and W₇C₃ carbides, forming (W,Mo)C and (W,Mo)₇C₃
• At moderate Mo (2–5 wt%): Mo₇C₃ carbides form preferentially, which are harder (2000–2500 HV) but more brittle than W₇C₃
• At high Mo (> 5 wt%): Complex carbides (Co₃W₃Mo₃C) form, with heterogeneous hardness and potential for microcracking

1. Precipitation hardening: Mo promotes the formation of fine M₆C carbide precipitates during cooling and subsequent aging, providing additional strengthening through Orowan mechanism.
2. Corrosion resistance enhancement: Mo improves resistance to reducing acids (H₂SO₄, HCl) and pitting corrosion by promoting the formation of MoO₃ in the passive film and increasing the pitting resistance equivalent number (PREN).
3. High-temperature strength retention: Mo maintains solid solution strengthening effectiveness at elevated temperatures (600–800°C) where other alloying elements may precipitate or segregate.

PTA Process Parameters and Their Interaction with Mo Content

Plasma transferred arc (PTA) cladding provides excellent control over dilution and microstructure due to its high energy density and focused heat input:

The dilution ratio (typically 10–25% for PTA cladding) directly affects the effective Mo content in the overlay deposit. For example, a powder with 5 wt% Mo applied with 20% dilution on a steel substrate (0% Mo) results in an effective Mo content of approximately 4.0 wt% in the overlay deposit.

Microstructural Evolution with Molybdenum Addition

Phase Composition and Distribution